Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)

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�

D

Cl3AlOCR
� � �

�[RC RCOð
O
B
ðð

š�O]š�q P

Acylium ions undergo electrophilic aromatic substitution
The acylium ion is suffi ciently electrophilic to attack benzene by the usual aromatic sub-
stitution mechanism.

H H

R �
�

�

�C
C

O

O H C

O)

Electrophilic Acylation

�kk

R

�

))

C
R

Because the newly introduced acyl substituent is electron withdrawing (see Sections 14-8
and 16-1), it deactivates the ring and protects it from further substitution. Therefore poly-
acylation does not occur, whereas polyalkylation does (Section 15-12). The effect is accen-
tuated by the formation of a strong complex between the aluminum chloride catalyst and
the carbonyl function of the product ketone.

Lewis Acid Complexation with 1-Phenylalkanones

AlCl3

AlCl3

�

�

�

C

O
B

ð
D

DC

O

R

D

R

))

1 5 - 1 3 F r i e d e l - C r a f t s A c y l a t i o n ( A l k a n o y l a t i o n )

Acetyl cation

MECHANISM

716 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y

This complexation removes the AlCl3 from the reaction mixture and necessitates the use of
at least one full equivalent of the Lewis acid to allow the reaction to go to completion.
Aqueous work-up is necessary to liberate the ketone from its aluminum chloride complex,
as illustrated by the following examples.

O

�HCl

1. AlCl3 (1.7 equivalents)
2. H2O, H�B

O
B

H
CH3CH2CCl

CCH2CH3

84%
1-Phenyl-1-acetone

(Propiophenone)

�

O

�CH3COOH

1. AlCl3 (2.4 equivalents)
2. H2O, H�B

O
B

O
B

H
CH3COCCH3

CCH3

85%
1-Phenylethanone
(Acetophenone)

�

The selectivity of the Friedel-Crafts acylation for single substitution allows for the
selective introduction of carbon chains into the benzene nucleus, a task that proved diffi cult
to accomplish by Friedel-Crafts alkylation (Section 15-12). Since we know how to convert
the carbonyl function into an alcohol by hydride reduction (Section 8-6) and the hydroxy
substituent into a leaving group that can be further reduced by hydride (Section 8-7), we
can synthesize the corresponding hydrocarbon. This sequence of acylation – reduction con-
stitutes a roundabout alkylation protocol that occurs selectively. We shall encounter more
direct “deoxygenations” of carbonyl groups later (Sections 16-5 and 17-10).

Cl�
AlCl3
�HCl

1. NaBH4
2. HBr

O
O

Br

Preparation of Hexylbenzene by Hexanoylation–Reduction of Benzene

LiAlH4

Hexylbenzene

Exercise 15-33

 O
 B
The simplest acyl chloride, formyl chloride, H – C – Cl, is unstable, decomposing to HCl and CO
upon attempted preparation. Therefore, direct Friedel-Crafts formylation of benzene is impossible.
An alternative process, the Gattermann-Koch reaction, enables the introduction of the formyl
group, – CHO, into the benzene ring by treatment with CO under pressure, in the presence of HCl
and Lewis acid catalysts. For example, methylbenzene (toluene) can be formylated at the para
position in this way in 51% yield. The electrophile in this process was observed directly for the
fi rst time in 1997 by treating CO with HF – SbF5 under high pressure:

13C NMR: d 139.5 ppm; IR:
n˜ 5 2110 cm21. What is the structure of this species and the mechanism of its reaction with

 C h a p t e r 1 5 717

In Summary The problems of Friedel-Crafts alkylation (multiple substitution and carboca-
tion rearrangements) are avoided in Friedel-Crafts acylations, in which an acyl halide or
carboxylic acid anhydride is the reaction partner, in the presence of a Lewis acid. The inter-
mediate acylium cations undergo electrophilic aromatic substitution to yield the corresponding
aromatic ketones.

THE BIG PICTURE
The concept of aromaticity (and antiaromaticity) may seem strange and new to you. However,
it is really just an extension of other electronic effects we have encountered earlier, starting
with Coulomb’s law (Section 1-2), the octet rule (Section 1-3), and the Aufbau principle
(Section 1-6). Other examples of orbital overlap and electron delocalization have been
shown to be either stabilizing or destabilizing, including the stability ordering of radicals
(Section 3-2) and carbocations (Section 7-5) due to hyperconjugation, and the general
 phenomenon of (de)stabilization by p-electron delocalization (Chapter 14). In this context,
aromaticity is simply another important kind of electronic effect in organic chemistry.

We shall explore additional implications of aromaticity in Chapter 16, showing how
single substituents on the benzene ring affect further substitutions. As a unit, the benzene
ring occurs in organic molecules ranging from polystyrene to aspirin; learning how to
incorporate benzene rings into other molecules and changing the substituents on a benzene
ring are important aspects of many branches of organic synthesis.

CHAPTER INTEGRATION PROBLEMS
15-34. Compound A, C8H10, was treated with Br2 in the presence of FeBr3 to give product B, C8H9Br.
The spectral data for A and B are given below. Assign structures to A and B.
 A: 1H NMR (CDCl3): d 5 2.28 (s, 6 H), 6.95 (m, 3 H), 7.11 (td, J 5 7.8, 0.4 Hz, 1 H) ppm.
 13C NMR (CDCl3): d 5 21.3, 126.1, 128.2, 130.0, 137.7 ppm.
 IR (neat) selected values: n˜ 5 3016, 2946, 2921, 769, 691 cm21.
 UV (CH3OH): lmax 5 261 nm.
 B: 1H NMR (CDCl3): d 5 2.25 (s, 3 H), 2.34 (s, 3 H), 6.83 (dd, J 5 7.9, 2.0 Hz, 1 H),

7.02 (dd, J 5 2.0, 0.3 Hz, 1 H), 7.36 (dd, J 5 7.9, 0.3 Hz, 1 H) ppm.
 13C NMR (CDCl3): d 5 20.7, 22.7, 121.6, 128.1, 131.6, 132.1, 136.9, 137.4 ppm.
 IR (neat) selected values: n˜ 5 3012, 2961, 2923 cm21.
 UV (CH3OH): lmax 5 265 nm.

SOLUTION
A cursory glance at the molecular formulas and the spectral data confi rms that bromination of a
substituted benzene is taking place: One hydrogen in A (C8H10) is replaced by a bromine atom
(C8H9Br), and both compounds show aromatic peaks in both the

1H and 13C NMR spectra. The IR
spectra confi rm this assignment by the presence of a Caromatic – H stretching signal for A and B, and
the UV spectra are consistent with a phenyl chromophore. A closer look reveals that both compounds
contain two methyl groups attached to the aromatic system at d < 2.3 ppm (Table 10-2). Subtracting
2 3 CH3 from C8H10 (A) leaves C6H4, a phenyl fragment: Compound A must be a dimethylbenzene
isomer. Consequently, B must be a bromo(dimethyl) benzene isomer. The question for both is:
Which isomer? To fi nd the answers, it is useful to write down all the possible options. For A,
there are three isomers: ortho-, meta-, and para-dimethylbenzene (xylene; Section 15-1).

methylbenzene? Explain the spectral data. (Hints: Draw the Lewis structure of CO and proceed
by considering the species that may arise in the presence of acid. The comparative spectral data
for free CO are 13C NMR: d 5 181.3 ppm; IR: n˜ 5 2143 cm21.)

CO HCl� �

CH3

H

CH3

CHO

AlCl3, CuCl

C h a p t e r I n t e g r a t i o n P r o b l e m s

718 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y

CH3

CH3

CH3

CH3

CH3

CH3

Ortho Meta Para
A

The Three Possible Structures for A

Is it possible to distinguish between them on the basis of NMR spectroscopy? The answer is yes,
because of the varying degrees of symmetry inherent to each ring. Thus, the ortho isomer should
exhibit only two types of aromatic hydrogens (two each) and three phenyl carbon signals in the
respective NMR spectra. The para isomer is even more symmetrical, containing only one type of
benzene hydrogen and two types of phenyl carbons. Both of these predictions are incompatible with
the observed data for A. Thus, while some of the aromatic hydrogens are unresolved and appear as
a multiplet (3 H), the presence of a single unique proton at d 5 7.11 ppm is consistent only with
meta-dimethylbenzene. (Which